Method and system for optoelectronics transceivers integrated on a cmos chip

ABSTRACT

Methods and systems for optoelectronics transceivers integrated on a CMOS chip are disclosed and may include receiving optical signals from optical fibers via grating couplers on a top surface of a CMOS chip, which may include a guard ring. Photodetectors may be integrated in the CMOS chip. A CW optical signal may be received from a laser source via grating couplers, and may be modulated using optical modulators, which may be Mach-Zehnder and/or ring modulators. Circuitry in the CMOS chip may drive the optical modulators. The modulated optical signal may be communicated out of the top surface of the CMOS chip into optical fibers via grating couplers. The received optical signals may be communicated between devices via waveguides. The photodetectors may include germanium waveguide photodiodes, avalanche photodiodes, and/or heterojunction diodes. The CW optical signal may be generated using an edge-emitting and/or a vertical-cavity surface emitting semiconductor laser.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application makes reference to and claims priority to U.S.Provisional Application Ser. No. 61/057,127 filed on May 29, 2008, andProvisional Application Ser. No. 60/997,298 filed on Oct. 2, 2007, whichare hereby incorporated herein by reference in their entirety.

This application is also a continuation in part of U.S. patentapplication Ser. No. 11/611,084 filed on Dec. 14, 2006.

This application also makes reference to U.S. Pat. Nos. 6,895,148;7,039,258; 7,245,803; and 7,366,380.

Each of the above stated applications and patents is hereby incorporatedherein by reference in its entirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[Not Applicable]

MICROFICHE/COPYRIGHT REFERENCE

[Not Applicable]

FIELD OF THE INVENTION

Certain embodiments of the invention relate to integrated circuit powercontrol. More specifically, certain embodiments of the invention relateto a method and system for optoelectronics transceivers integrated on aCMOS chip.

BACKGROUND OF THE INVENTION

As data networks scale to meet ever-increasing bandwidth requirements,the shortcomings of copper data channels are becoming apparent. Signalattenuation and crosstalk due to radiated electromagnetic energy are themain impediments encountered by designers of such systems. They can bemitigated to some extent with equalization, coding, and shielding, butthese techniques require considerable power, complexity, and cable bulkpenalties while offering only modest improvements in reach and verylimited scalability. Free of such channel limitations, opticalcommunication has been recognized as the successor to copper links.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with the present invention as set forth inthe remainder of the present application with reference to the drawings.

BRIEF SUMMARY OF THE INVENTION

A system and/or method for optoelectronics transceivers integrated on aCMOS chip, substantially as shown in and/or described in connection withat least one of the figures, as set forth more completely in the claims.

Various advantages, aspects and novel features of the present invention,as well as details of an illustrated embodiment thereof, will be morefully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a block diagram of a photonically enabled CMOS chip, inaccordance with an embodiment of the invention.

FIG. 1B is a diagram illustrating an oblique view of an exemplaryphotonically enabled CMOS chip, in accordance with an embodiment of theinvention.

FIG. 1C is a diagram illustrating an exemplary CMOS chip coupled to anoptical fiber cable, in accordance with an embodiment of the invention.

FIG. 2A is a block diagram of an exemplary split domain Mach-Zehndermodulator, in accordance with an embodiment of the invention.

FIG. 2B is a cross-sectional schematic of exemplary phase modulators, inaccordance with an embodiment of the invention.

FIG. 3 is a block diagram of an exemplary photonically-enabled processflow, in accordance with an embodiment of the invention.

FIG. 4 is a cross-section of exemplary integrated electrical andoptoelectronic devices, in accordance with an embodiment of theinvention.

FIG. 5 is a diagram illustrating cross-sections of exemplary germaniumphotodiodes, in accordance with an embodiment of the invention.

FIG. 6 is a diagram illustrating an exemplary grating coupler, inaccordance with an embodiment of the invention.

FIG. 7 is a diagram illustrating exemplary optical devices, inaccordance with an embodiment of the invention.

FIG. 8 is a diagram illustrating an exemplary waveguide avalanchephotodiode, in accordance with an embodiment of the invention.

FIG. 9 is a diagram illustrating an exemplary optical source assembly,in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Certain aspects of the invention may be found in a method and system foroptoelectronics transceivers integrated on a CMOS chip. Exemplaryaspects of the invention may comprise receiving optical signals from oneor more optical fibers via grating couplers on a top surface of a CMOSchip. The optical signals may be converted to electrical signals via oneor more photodetectors integrated in or on the CMOS chip. The electricalsignals may be processed via circuitry in the CMOS chip. Acontinuous-wave (CW) optical signal may be received from a laser sourcevia grating couplers on the top surface of the CMOS chip, and may bemodulated via one or more optical modulators integrated in or on theCMOS chip. Electrical signals may be received via the circuitry in theCMOS chip, and may drive the one or more optical modulators. Themodulated optical signal may be communicated out of the top surface ofthe CMOS chip into one or more optical fibers via one or more gratingcouplers integrated in or on the CMOS chip. The received CW opticalsignal may be modulated via Mach-Zehnder modulators and/or ringmodulators, for example. The CMOS chip may comprise a CMOS guard ring.The received optical signals may be communicated between devices on theCMOS chip via waveguides. The one or more photodetectors may comprisegermanium waveguide photodiodes, germanium waveguide avalanchephotodiodes, and/or heterojunction diodes, for example. The CW opticalsignal may be generated using an edge-emitting semiconductor laserand/or a vertical-cavity surface emitting semiconductor laser, which maybe bonded to the top surface of the CMOS chip, for example. The one ormore modulators may be driven using circuitry on the CMOS chip.

FIG. 1A is a block diagram of a photonically enabled CMOS chip, inaccordance with an embodiment of the invention. Referring to FIG. 1A,there is shown optoelectronic devices on a CMOS chip 130 comprising highspeed optical modulators 105A-105D, high-speed photodiodes 111A-111D,monitor photodiodes 113A-113H, and optical devices comprising taps103A-103K, optical terminations 115A-115D, and grating couplers117A-117H. There is also shown electrical devices and circuitscomprising transimpedance and limiting amplifiers (TIA/LAs) 107A-107E,analog and digital control circuits 109, and control sections 112A-112D.Optical signals are communicated between optical and optoelectronicdevices via optical waveguides fabricated in the CMOS chip 130.Additionally, optical waveguides are indicated in FIG. 1A by the dashedovals.

The high speed optical modulators 105A-105D comprise Mach-Zehnder orring modulators, for example, and enable the modulation of the CW laserinput signal. The high speed optical modulators 105A-105D are controlledby the control sections 112A-112D, and the outputs of the modulators areoptically coupled via waveguides to the grating couplers 117E-117H. Thetaps 103D-103K comprise four-port optical couplers, for example, and areutilized to sample the optical signals generated by the high speedoptical modulators 105A-105D, with the sampled signals being measured bythe monitor photodiodes 113A-113H. The unused branches of the taps103D-103K are terminated by optical terminations 115A-115D to avoid backreflections of unwanted signals.

The grating couplers 117A-117H comprise optical gratings that enablecoupling of light into and out of the CMOS chip 130. The gratingcouplers 117A-117D are utilized to couple light received from opticalfibers into the CMOS chip 130, and may comprise polarization independentgrating couplers. The grating couplers 117E-117H are utilized to couplelight from the CMOS chip 130 into optical fibers. The optical fibers maybe epoxied, for example, to the CMOS chip, and may be aligned at anangle from normal to the surface of the CMOS chip 130 to optimizecoupling efficiency.

The high-speed photodiodes 111A-111D convert optical signals receivedfrom the grating couplers 117A-117D into electrical signals that arecommunicated to the TIA/LAs 107A-107D for processing. The analog anddigital control circuits 109 may control gain levels or other parametersin the operation of the TIA/LAs 107A-107D. The TIA/LAs 107A-107D maythen communicate electrical signals to other circuitry on the CMOS chip130 and/or circuitry/devices off-chip.

The TIA/LAs 107A-107D may comprise narrowband, non-linear optoelectronicreceiver circuitry. Accordingly, the narrowband receiver front-end maybe followed by a non-return to zero (NRZ) level restorer circuit. Thiscircuit limits the bandwidth of the optical receiver in order todecrease the integrated noise, thereby increasing the signal to noiseratio. An NRZ level restorer may be used to convert the resulting datapulses back into NRZ data.

The control sections 112A-112D comprise electronic circuitry that enablemodulation of the CW laser signal received from the taps 103A-103C. Thehigh speed optical modulators 105A-105D require high-speed electricalsignals to modulate the refractive index in respective branches of aMach-Zehnder interferometer (MZI), for example. The voltage swingrequired for driving the MZI is a significant power drain in the CMOSchip 130. Thus, if the electrical signal for driving the modulator maybe split into domains with each domain traversing a lower voltage swing,power efficiency is increased.

In an embodiment of the invention, the integration of all optical,electrical and optoelectronic devices required for a transceiver, alongwith a coupled laser source, enables the integration of multipleoptoelectronic transceivers on a single chip. In an exemplaryembodiment, the CMOS chip 130 comprises four optoelectronic transceiverswith one optical source, and enables communication of optical signalsvertically to and from the chip surface, thus enabling the use of CMOSprocesses, including a CMOS guard ring, as discussed with respect toFIGS. 1B and 1C.

FIG. 1B is a diagram illustrating an oblique view of an exemplaryphotonically enabled CMOS chip, in accordance with an embodiment of theinvention. Referring to FIG. 1B, there is shown the CMOS chip 130comprising electronic devices/circuits 131, optical and optoelectronicdevices 133, a light source interface 135, CMOS chip surface 137, anoptical fiber interface 139, and CMOS guard ring 141.

The light source interface 135 and the optical fiber interface 139comprise grating couplers that enable coupling of light signals via theCMOS chip surface 137, as opposed to the edges of the chip as withconventional edge-emitting devices. Coupling light signals via the CMOSchip surface 137 enables the use of the CMOS guard ring 141 whichprotects the chip mechanically and prevents the entry of contaminantsvia the chip edge.

The electronic devices/circuits 131 comprise circuitry such as theTIA/LAs 107A-107D and the analog and digital control circuits 109described with respect to FIG. 1A, for example. The optical andoptoelectronic devices 133 comprise devices such as the taps 103A-103K,optical terminations 115A-115D, grating couplers 117A-117H, high speedoptical modulators 105A-105D, high-speed photodiodes 111A-111D, andmonitor photodiodes 113A-113H.

FIG. 1C is a diagram illustrating an exemplary CMOS chip coupled to anoptical fiber cable, in accordance with an embodiment of the invention.Referring to FIG. 1C, there is shown the CMOS chip 130 comprising theelectronic devices/circuits 131, the optical and optoelectronic devices133, the CMOS chip surface 137, and the CMOS guard ring 141. There isalso shown a fiber to chip coupler 143, an optical fiber cable 145, anda light source module 147.

The CMOS chip 130 comprising the electronic devices/circuits 131, theoptical and optoelectronic devices 133, the CMOS chip surface 137, andthe CMOS guard ring 141 may be as described with respect to FIG. 1B.

In an embodiment of the invention, the optical fiber cable may beaffixed, via epoxy for example, to the CMOS chip surface 137. The fiberchip coupler 143 enables the physical coupling of the optical fibercable 145 to the CMOS chip 130.

The light source module 147 may be affixed, via epoxy or solder, forexample, to the CMOS chip surface 137. In this manner a high power lightsource may be integrated with optoelectronic and electronicfunctionalities of one or more high-speed optoelectronic transceivers ona single CMOS chip.

FIG. 2A is a block diagram of an exemplary split domain Mach-Zehndermodulator, in accordance with an embodiment of the invention. Referringto FIG. 2A, there is shown a split-domain Mach-Zehnder modulator (MZM)250 comprising a transmission line driver 209, waveguides 211,transmission lines 213A-213D, diode drivers 215A-215H, diodes 219A-219D,and transmission line termination resistors R_(TL1)-R_(TL4). There isalso shown voltage levels V_(dd), V_(d), and Gnd. In an embodiment ofthe invention, V_(d) is equal to a voltage of V_(dd)/2, thus generatingtwo voltage domains, due to the symmetric nature of the stackedcircuits. However, the invention is not limited to two voltage domains.Accordingly, any number of voltage domains may be utilized, dependent onthe desired voltage swing of each domain and the total voltage range,defined here as V_(dd) to ground. Similarly, the magnitude of thevoltage range in each voltage domain may be a different value than otherdomains.

The transmission line (T-line) driver 209 comprises circuitry fordriving transmission lines in an even-coupled mode, where the signal oneach pair of transmission lines is equal except with a DC offset. Inthis manner, two or more voltage domains may be utilized to drive thediodes that generate index changes in the respective branches of the MZM250. In another embodiment of the invention, the T-line driver 209 maydrive transmission lines in odd-coupled mode. Even-coupled mode mayresult in a higher impedance in the transmission line, whereasodd-coupling may result in lower impedance.

The waveguides 211 comprise the optical components of the MZM 250 andenable the routing of optical signals around the CMOS chip 130. Thewaveguides 211 comprise silicon and silicon dioxide, formed by CMOSfabrication processes, utilizing the index of refraction differencebetween Si and SiO₂ to confine an optical mode in the waveguides 211.The transmission line termination resistors R_(TL1)-R_(TL4) enableimpedance matching to the T-lines 213A-213D and thus reducedreflections.

The diode drivers 215A-215H comprise circuitry for driving the diodes219A-219D, thereby changing the index of refraction locally in thewaveguides 211. This index change in turn changes the velocity of theoptical mode in the waveguides 211, such that when the waveguides mergeagain following the driver circuitry, the optical signals interfereconstructively or destructively, thus modulating the laser input signal.By driving the diodes 219A-219D with a differential signal, where asignal is driven at each terminal of a diode, as opposed to one terminalbeing tied to AC ground, both power efficiency and bandwidth may beincreased due to the reduced voltage swing required in each domain.

In operation, a CW optical signal is coupled into the “Laser Input”, anda modulating differential electrical signal is communicated to theT-line driver 209. The T-line driver 209 generates complementaryelectrical signals to be communicated over the T-lines 213A-213D, witheach pair of signals offset by a DC level to minimize the voltage swingof each diode driver 215A-215H, while still enabling a full voltageswing across the diodes 219A-219D.

Reverse biasing the diodes 219A-219D modifies the width of the depletionregion, modulating the carrier density overlap with the optical mode,and thus the speed of the optical signal propagating through thewaveguides 213A-213D. The optical signals then interfere constructivelyor destructively, resulting in the “Modulated Light” signal.

In an embodiment of the invention, the MZM 250 is integrated on the chip130, enabling the integration of one or more transceivers on a singleCMOS chip, whereas conventional transceivers utilize discrete optical,electrical, and optoelectronic devices and chips. By integratingoptoelectronic transceivers on a single chip, power and spacerequirements may be greatly reduced.

FIG. 2B is a cross-sectional schematic of exemplary phase modulators, inaccordance with an embodiment of the invention. Referring to FIG. 2B,there is shown a PIN phase modulator 260 and a thermal phase modulator(TPM) 270. The PIN phase modulator 260 comprises an n region 251, anintrinsic region 253, a p region 255, an n+ region 257, a p+ region 259,and contacts 261A and 261B. The TPM 270 comprises n region 251A and251B, an n− region 263, n+ regions 257A and 257B, and contact layers261A and 261B.

The n region 251 comprises a region of n-type silicon to define then-side of a PIN junction. The intrinsic region 253 comprises a region ofunintentionally doped silicon, forming the i-region of a PIN junction.The p region 255 comprises p-doped silicon, forming the p-region of aPIN junction. The n+ region 257 and the p+ region 259 compriselow-resistivity doped silicon to enable ohmic contact to the PIN diodecomprising the p region 255, the intrinsic region 253 and the n region251. The contact layers 261A and 261B comprise metal or otherlow-resistivity material to enable an ohmic contact to the underlyingsemiconductor material.

The n regions 251A and 251B comprises regions of n-type silicon todefine the n-sides of an n/n−/n resistive device. The n− region 263 maycomprise a low n-doped silicon region to provide a resistive structure.The n+ regions 257A and 257B comprise highly-doped n-type silicon toenable ohmic contact to the lower n-doped material. The contact layers261A and 261B comprise metal or other low-resistivity material to enablean ohmic contact to the underlying semiconductor material.

In operation, the PIN phase modulator 260 performance may be controlledby an electrical signal applied to the contacts 261A and 261B. Inhigh-speed applications, the PIN phase modulator 260 may be reversebiased, which modifies the width of the depletion region, modulating thecarrier density overlap with the optical mode, and thus the speed of theoptical mode propagating through the PIN phase modulator 260.Alternatively, for lower speed applications the PIN phase modulator 260may be forward biased for carrier injection mode, again affecting theindex of refraction for the mode propagating through the PIN phasemodulator 260. This mode may exhibit very high efficiency, but also highdynamic loss.

In another embodiment of the invention, the PIN phase modulator 260 maybe used as a variable optical attenuator by forward biasing thestructure, causing increased optical mode loss.

The TPM 270 may modulate an optical signal, the optical mode, by flowingcurrent through a resistor defined by the n regions 251A and 251B, andthe n− region 263, thereby providing electrical loss, and thus localheating. This heating alters the index of refraction of the waveguideessentially defined by the n− region of the TPM 270. This structure mayexhibit relatively low efficiency, but also low dynamic loss.

FIG. 3 is a block diagram of an exemplary photonically-enabled processflow, in accordance with an embodiment of the invention. Referring toFIG. 3, there is shown a photonically enabled process flow 300comprising an original CMOS process 320, a shallow trench module 303, acustom implant module 307, source/drain implants module 313, a blockingsalicidation module 315, a germanium module 317, and a data processingmodule 323.

The original CMOS process comprises a beginning of process flow step301, a deep trench module 305, a well module 309, a gate module 311, aback-end metals module 319, and a wafers out step 321.

The photonically-enabled process flow begins with custom SOI substrateswith appropriate oxide thickness for optical processes being insertedinto the beginning of process flow step 301 which may comprise suitablewafer preparation processes, such as sorting, cleaning, or qualitycontrol, for example. The wafers then proceed to the shallow trenchmodule 303 for defining and etching of shallow trenches. The shallowtrench module 303 may comprise photolithography, etching, fill, andchemical-mechanical polishing (CMP), for example, followed by the deeptrench module 305, which comprises a conventional CMOS trench module.

The well module 309 comprises photolithography steps and dopant ionimplantation to define the wells for CMOS devices. The custom implantsmodule 307 may be inserted into the well module 309, to define dopingregions specific to optoelectronic devices, for example. The wafers thenproceed to the gate module 311 to define CMOS gates viaphotolithography, etching, trimming, spacers, and implants, for example.Source and drain implants may be performed by the source/drain implantmodule 313 before proceeding to the blocking salicidation module 315.

The salicidation module 315 generates a self-aligned silicide layer inthe silicon surface for metal contacts, which is followed by thegermanium module 317, which may deposit germanium for integratedphotodetectors on the SOI silicon wafers. In an embodiment of theinvention, the germanium process may be fully CMOS compatible. Thewafers then proceed to the back-end metals module 319 which comprises a6-metal low-k copper process, for example, followed by the wafers outstep 321.

In an exemplary embodiment of the invention, the CMOS process flowcomprises a 0.13 micron CMOS SOI technology platform for integratingguided-wave optics. The photolithography process comprises deep-UVtechnology to enable near-IR optics capability, and the high resistivitysubstrate may enable low microwave loss in the circuitry. Custom stepsmay be utilized in standard tools, and comprise silicon etch andimplant, germanium epitaxy, and may utilize a standard contact module.These processes may be thermal budget compliant, and require no postprocessing.

FIG. 4 is a cross-section of exemplary integrated electrical andoptoelectronic devices, in accordance with an embodiment of theinvention. Referring to FIG. 4, there is shown an integratedsemiconductor structure 400 comprising a transistor 410 and a modulator420 and associated layers. The layers are utilized to fabricate thetransistor 410 and the modulator 420, to isolate, and to provideelectrical connection to the devices, for example.

The integrated semiconductor structure 400 comprises a silicon substrate401, a buried oxide 403, a field oxide 405, a contact layer 415, a metal1 layer 417, a via 1 layer 419, a metal 2 layer 421, a last metal layer423, a passivation layer 425, and a metal cap 427. The metal 1 layer417, the metal 2 layer 421, the last metal layer, and the metal cap 427provide electrical contact between layers and to electrical andoptoelectronics devices, such as the transistor 410 and the modulator420. The via 1 layer 419 and the contact layer 415 also enableelectrical contact to the devices while providing electrical isolationbetween devices by incorporating insulating materials between conductivevias.

The transistor 410 comprises the well 407, drain and source implantlayers 429A and 429B, respectively, a gate 431, and a passivation layer433. The well region 407 is a doped silicon layer that enablescomplementary devices to be integrated on the same substrate by creatinga layer of opposite doping to that of the region outside the well. Inthis manner, in instances where the well is n-doped, the source anddrain implant layers 429A and 429B may comprise p-doped silicon, forexample.

The gate 431 may comprise metal or polysilicon, for example, that may beisolated from the well 407 by a thin oxide layer (not shown). The gatemay be passivated by the passivation layer 433, which may provideelectrical isolation from other metal layers, for example.

The modulator 420 comprises a p-region 409, an n-region 411, a salicideblock 413, and etched region 439. The salicide block 413 comprises alayer of material to prevent the silicon of the modulator 420 and otheroptical devices from being salicided during the standard CMOS process.If silicon in the optical devices was salicided, large optical losseswould result. Additionally, the salicide block 413 blocks unwantedimplants into the waveguides and other optical devices, which would alsocause unwanted loss.

FIG. 5 is a diagram illustrating cross-sections of exemplary germaniumphotodiodes, in accordance with an embodiment of the invention.Referring to FIG. 5, there is shown a germanium homojunction photodiode500 and a germanium single heterojunction (SH) waveguide photodiode 520.The germanium homojunction waveguide photodiode 500 comprises a siliconsubstrate 501A, a Ge layer 503A, a p-doped Ge (p-Ge) layer 505, ann-doped Ge (n-Ge) layer 507, an anode contact 513A, and a cathodecontact 515A. The anode contact 513A and the cathode contact 515A maycomprise metal or other conductive material that enables electricalcontact to the homojunction waveguide photodiode 500. The p-Ge layer505, the Ge layer 503A, and the n-Ge layer 507 comprise a homojunctiondiode that may be enabled to detect an optical signal and generate anelectrical output signal.

The SH photodiode 520 comprises a silicon substrate 501B, Ge layer 503B,a p-Ge layer 509, an n-doped silicon (n-Si) layer 511, an anode contact513B, and a cathode contact 515B. The p-Ge layer 509, the Ge layer 503B,the Si substrate 501B, and the n-Si layer 511 comprise a singleheterojunction diode that may be enabled to detect an optical signal andgenerate an electrical output signal. A heterojunction photodiode maydemonstrate lower dark currents than a homojunction device, and awaveguide detector may enable higher quantum efficiencies due to theincreased absorption length in the lateral direction compared to avertical photodiode. In another embodiment of the invention, thephotodiode may comprise a double heterojunction photodiode defined bytwo Si—Ge junctions. In an embodiment of the invention, one or more Gewaveguide photodiodes may be integrated in a CMOS chip, such as the CMOSchip 130 described with respect to FIG. 1A-1C, enabling the integrationof one or more optoelectronic transceivers on the chip.

FIG. 6 is a diagram illustrating an exemplary grating coupler, inaccordance with an embodiment of the invention. Referring to FIG. 6,there is shown a grating coupler 600 comprising a Si substrate 601, aburied oxide 603, a waveguide layer 605, a dielectric stack 607, agrating 609, an optical mode 611, an emitted optical mode 613, and areflected optical mode 615. A plurality of grating couplers, such as thegrating coupler 600, may be integrated in the chip 130, described withrespect to FIG. 1A-1C.

The waveguide layer 605 comprises a layer of silicon, for example, thatsupports an optical mode 611 that may be communicated from other opticaland/or optoelectronic devices in the chip 130. The grating 609 comprisesa pattern of alternating dielectric constant material that is configuredto transmit the optical mode 613 out of the surface of the chip 130 viathe dielectric stack 607. The dielectric stack 607 may be configured toenable the transmission of the scattered optical signal out of the topsurface, at an angle from vertical.

In operation, an optical mode 611 may be scattered out of the topsurface of the chip 130 via the grating coupler 600. In an exemplaryembodiment, loss coupling may be 1.5 dB or lower, for example, and mayenable wafer-scale testing. In an embodiment of the invention, thegrating coupler may enable 20× mode-size conversion laterally andlongitudinally.

FIG. 7 is a diagram illustrating exemplary optical devices, inaccordance with an embodiment of the invention. Referring to FIG. 7,there is shown a grating coupler 700, a waveguide bend 710, a Y-junction720, a waveguide termination 730, a directional coupler 740, apolarization splitting grating coupler 750, a waveguide 760, and awaveguide taper 770. The grating coupler 700 may be substantiallysimilar to the grating coupler 600 described with respect to FIG. 6.

The waveguide 760 comprises a silicon waveguide that may be enabled tocommunicate an optical mode with minimal optical loss. The waveguide 760may comprise a single-mode waveguide or a multi-mode waveguide. Asingle-mode waveguide may have higher propagation loss, but may enabletight bends with a 20 micron radius, such as the waveguide bend 710, forexample, or circular rings. A multi-mode waveguide may have lower loss,and thus may be used for communicating optical signals over largerdistances.

The Y-junction 720 may comprise a waveguide that splits into twoseparate waveguides, thus enabling signal splitting and/or signalcombining. The Y-junction 720 may be wavelength and fabricationinsensitive. The directional coupler 740 may comprise two opticalwaveguides that converge to a small distance apart, and then divergeagain, thus enabling coupling and/or splitting of optical signals. Theminimum spacing between the waveguides may determine the couplingefficiency of the directional coupler.

The waveguide termination 730 comprises an absorbing grating structurecomprising a grating/metal/Ge structure, fabricated on a waveguideadjacent to a waveguide taper, such as the waveguide taper 770. Thegrating fabricated on the tapered waveguide may absorb the optical modetraveling into the waveguide termination 730, thus mitigating reflectedoptical signals, which may degrade performance.

The polarization splitting grating coupler (PSGC) 750 may enable thecoupling of an incoming light signal and generating two output signalscommunicated to other optical components on the chip. The PSGC 750 mayseparate an incoming optical signal into two orthogonal output signalsin the same TE waveguide mode. The input/output comprises anypolarization, and may enable a simple polarization-diversity scheme.

FIG. 8 is a diagram illustrating an exemplary waveguide avalanchephotodiode, in accordance with an embodiment of the invention. Referringto FIG. 8, there is shown a germanium waveguide avalanche photodiode (GeWG-APD) 800 integrated on a CMOS chip, such as the CMOS chip 130described with respect to FIG. 1A-1C. The Ge WG APD 800 may befabricated on a silicon substrate 801 and buried oxide layer 803. The GeWG-APD 800 comprises a Si layer 805, a Ge layer 807, a p-doped Ge (p-Ge)layer 809, n-doped silicon (n-Si) layers 811A and 811B, salicided Silayers 813A and 813B, anodes 815, cathodes 817A and 817B, an opticalmode 819, and an avalanche region 821.

The salicided Si layers 813A and 813B comprise silicon layers that havebeen salicided to enable a good electrical contact to the underlyinglower, or undoped, Si layer 805. The avalanche region 821 comprises thehigh electric region defined by the Ge layer 807 and the n-Si layers811A and 811B.

In operation, the optical mode 819 may be substantially confined to theGe layer 807 and may be absorbed creating electron-hole pairs. Theelectric field generated by applying a large reverse bias across theanode and cathodes separates the carriers. With appropriate bias, theelectric field in the avalanche region 821 may be high enough toinitiate an avalanche process, multiplying the number of photogeneratedcarriers by a multiplier, M, which may result in a higher responsivitythan a PIN photodiode.

FIG. 9 is a diagram illustrating an exemplary optical source assembly,in accordance with an embodiment of the invention. Referring to FIG. 9,there is shown an optical source assembly 900 comprising an opticalbench 901, a laser mount 903, a laser diode 905, a ball lens 907, anoptical isolator 909, a mirror element 911, bottom bond pads 913, topcontact pads 915, a pyrex/quartz lid 917, and a support ring 919.

In an embodiment of the invention, the pyrex/quartz lid 917 comprises apyrex/quartz stack which may be orientated either way, with the pyrex orthe quartz on top, and enables hermetic sealing of the opticalcomponents to improve device lifetime. The optical isolator 909 mayenable the reduction of reflected signals back to the laser diode 905.In an alternative embodiment of the invention, the optical isolatorcomprises a faraday rotator.

The optical bench 901 comprises a silicon optical bench, for example,and is micromachined and/or etched to comprise recessed features toaffix devices such as the isolator 909 and the ball lens 907, forexample. The support ring 919, comprising etched silicon, for example,provides hermetic sealing in conjunction with the pyrex/quartz lid 917and the optical bench 910. The mirror element 911 may comprise areflective surface deposited on the support ring 919. In anotherembodiment of the invention, the mirror element 911 may be fabricated byetching and/or polishing the surface of the support ring 919. In anembodiment of the invention, the support ring comprises silicon whichmay be micromachined, such as by anisotropic etching, for example, tocreate an optically reflective surface for the mirror element 911.

The laser diode 905 comprises an edge-emitting semiconductor laser, forexample, and may be enabled to emit an optical signal of a desiredwavelength. The laser mount 903 provides mechanical support and heatsinking capability for the laser diode 905. The ball lens 907 focusesthe optical signal generated by the laser diode 905 on to the isolated909.

In an embodiment of the invention, by integrating the electrical andoptical interface on the same side of the wafer, wafer-scale fibercoupling testing of the laser package may be enabled. The bottom sidepad facilitates wire bonding after attachment to the CMOS die, which maybe coupled to the side where the Light Out optical signal emerges. Amembrane may be incorporated into the silicon bench or alternately oneor more thermistors may be fabricated in the package as a hermeticitysensor.

In operation, an optical signal may be generated by the laser diode 905which is focused and/or collimated by the ball lens 907 onto theisolator 909. The light signal is then reflected out of the opticalsource assembly 900 via the mirror element 911, generating the LightOut.

In an embodiment of the invention, a method and system are disclosed forreceiving optical signals from one or more optical fibers via gratingcouplers on a top surface of a CMOS chip 130. The optical signals may beconverted to electrical signals via one or more photodetectors 111A-111Dintegrated in or on the CMOS chip 130. The electrical signals may beprocessed using circuitry in the CMOS chip 107A-107D/109. Acontinuous-wave (CW) optical signal may be received from a laser source147 via grating couplers on the top surface 137 of the CMOS chip 130,and may be modulated via one or more optical modulators 105A-105Dintegrated in or on the CMOS chip 130. Electrical signals may bereceived via the circuitry in the CMOS chip 130, and may drive the oneor more optical modulators 105A-105D. The modulated optical signal maybe communicated out of the top surface 137 of the CMOS chip 130 into oneor more optical fibers via one or more grating couplers 117E-117Hintegrated in or on the CMOS chip 130. The received CW optical signalmay be modulated using Mach-Zehnder modulators and/or ring modulators.The CMOS chip 130 may comprise a CMOS guard ring 141. The receivedoptical signals may be communicated between devices on the CMOS chip viawaveguides. The one or more photodetectors 111A-111D/113A-113H maycomprise germanium waveguide photodiodes, germanium waveguide avalanchephotodiodes, and/or heterojunction diodes, for example. The CW opticalsignal may be generated via an edge-emitting semiconductor laser and/ora vertical-cavity surface emitting semiconductor laser, which may bebonded to the top surface 137 of the CMOS chip 130, for example. The oneor more modulators 105A-105D may be driven using circuitry on the CMOSchip 130.

While the invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiments disclosed, but that the present inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A method for processing signals, the method comprising: in a CMOSchip, receiving optical signals from one or more optical fibers viagrating couplers on a top surface of said CMOS chip; converting saidoptical signals to electrical signals via one or more photodetectorsintegrated in said CMOS chip; processing said electrical signals usingcircuitry in said CMOS chip; receiving a continuous-wave (CW) opticalsignal from a laser source via grating couplers on said top surface ofsaid CMOS chip; modulating said received CW optical signal using one ormore optical modulators integrated in said CMOS chip; receivingelectrical signals from circuitry in said CMOS chip; driving said one ormore optical modulators using said received electrical signals; andcommunicating said modulated optical signal out of said top surface ofsaid CMOS chip into one or more optical fibers via one or more gratingcouplers integrated in said CMOS chip.
 2. The method according to claim1, comprising modulating said received CW optical signal usingMach-Zehnder modulators.
 3. The method according to claim 1, comprisingmodulating said received CW optical signal using ring modulators.
 4. Themethod according to claim 1, wherein said CMOS chip comprises a CMOSguard ring.
 5. The method according to claim 1, comprising communicatingsaid received optical signals between devices on said CMOS chip viawaveguides.
 6. The method according to claim 1, wherein said one or morephotodetectors comprise germanium waveguide photodiodes.
 7. The methodaccording to claim 1, wherein said wherein said one or morephotodetectors comprise germanium waveguide avalanche photodiodes. 8.The method according to claim 1, wherein said wherein said one or morephotodetectors comprise heterojunction diodes.
 9. The method accordingto claim 1, comprising generating said CW optical signal using anedge-emitting semiconductor laser.
 10. The method according to claim 1,comprising generating said CW optical signal using a vertical-cavitysurface emitting semiconductor laser.
 11. The method according to claim1, wherein said laser source is bonded to said top surface of said CMOSchip.
 12. A system for processing signals, the system comprising: a CMOSchip comprising: one or more grating couplers on a top surface of saidCMOS chip and enabled to receive optical signals from one or moreoptical fibers; one or more photodetectors integrated in said CMOS chipand enabled to convert said received optical signals to electricalsignals; one or more circuits integrated in said CMOS chip and enabledto process said electrical signals; one or more grating couplers on saidtop surface of said CMOS chip and enabled to receive a continuous-wave(CW) optical signal from a laser source; one or more optical modulatorsintegrated in said CMOS chip and enabled to modulate said received CWoptical signal; one or more circuits integrated in said CMOS chip andenabled to receive electrical signals and drive said one or more opticalmodulators using said received electrical signals; and one or moregrating couplers integrated in said CMOS chip and enabled to communicatesaid modulated optical signal out of said top surface of said CMOS chipinto one or more optical fibers.
 13. The system according to claim 12,wherein said optical modulators comprise Mach-Zehnder modulators. 14.The system according to claim 12, wherein said optical modulatorscomprise ring modulators.
 15. The system according to claim 12, whereinsaid CMOS chip comprises a CMOS guard ring.
 16. The system according toclaim 12, wherein said received optical signals are communicated betweendevices on said CMOS chip using waveguides.
 17. The system according toclaim 12, wherein said one or more photodetectors comprise germaniumwaveguide photodiodes.
 18. The system according to claim 12, whereinsaid wherein said one or more photodetectors comprise germaniumwaveguide avalanche photodiodes.
 19. The system according to claim 12,wherein said wherein said one or more photodetectors compriseheterojunction diodes.
 20. The system according to claim 12, whereinsaid laser source is an edge-emitting semiconductor laser.
 21. Thesystem according to claim 12, wherein said laser source is avertical-cavity surface emitting semiconductor laser.
 22. The systemaccording to claim 12, wherein said laser source is bonded to said topsurface of said CMOS chip.